Chromosomal translocations are common genetic abnormalities found in both leukemias and solid tumors. These rearrangements often lead to the activation of proto-oncogenes or production of tumor- specific fusion proteins. While much has been learned about the effects of specific translocations on cell proliferation, much less is known about what causes these chromosome rearrangements. While a subset of translocations found in lymphoid malignancies may involve the RAG recombination system, the mechanistic basis for most other translocations remains unknown. One reason for this lack of understanding about the development of translocations compared to other cancer-associated genetic abnormalities (e.g. mismatch repair and nucleotide excision repair deficiencies, aneuploidy, etc.) is the lack of a genetic selection system for translocations in a model organism. This proposal describes the development and use of a system that genetically selects for rare translocations, of the types seen in human tumors, using the yeast Saccharomyces cerevisiae. A set of yeast artificial chromosomes has been constructed that contain chosen sequences of human DNA flanked by genetic markers. In the appropriate yeast strain, translocations whose breakpoints occur within the human DNA sequences can be selected for under a wide variety of growth conditions. This system will be used to answer both specific questions, such as whether DNA topoisomerase II participates in translocations involving the MLL gene, and general questions about what other DNA metabolic or cell cycle checkpoint gene products are required to prevent or cause chromosomal translocations.